1) Field of the Invention
The invention relates to a process for preparing adhesives having improved adhesion and to the use of the adhesives obtainable by this process in adhesive compositions for paper, packaging, wood and textiles and in structural adhesives.
2) Background Art
Polyvinyl alcohol-stabilized vinyl acetate-ethylene copolymers in the form of their aqueous dispersions are frequently employed to bond paper and packaging materials. A disadvantage, however, is that the adhesion is often inadequate to bond to surfaces of plastic. To improve the adhesion, it is common to add plasticizing additives to modify the adhesive with polyacrylate dispersion or to lower the glass transition temperature of the vinyl acetate-ethylene copolymer by incorporating larger amounts of ethylene into it. A disadvantage of these measures, however, is the marked reduction in the cohesion of the adhesives.
It is therefore an object of the invention to improve the adhesion of adhesives, especially those based on polyvinyl alcohol-stabilized vinyl acetate-ethylene copolymers, without having to deal with the above-mentioned disadvantages.
It has surprisingly been found that by copolymerization with vinyl esters of alpha-branched tertiary monocarboxylic acids of 11 carbon atoms, the adhesion of the adhesives can be increased considerably without a dramatic drop in the cohesion.
The use of vinyl esters of alpha-branched tertiary monocarboxylic acids to prepare terpolymer dispersions with vinyl acetate and ethylene is known. The patent literature to date has described the use of vinyl esters of alpha-branched carboxylic acids of 5, 9 or 10 carbon atoms (VeoVa5(copyright), VeoVa9(copyright) and VeoVa10(copyright), trade names of Shell), especially for improving the alkali stability and light stability, and for reducing the water absorption of the polymers.
JP-A 07113069 (Derwent Abstract AN 95-196794) discloses VeoVa9(copyright)/ethylene copolymer dispersions for bonding polypropylene surfaces and JP-A 05025449 (Derwent Abstract 93-080621) discloses aqueous VeoVa10(copyright)/ethylene copolymer dispersions for bonding polyethylene foams. JP-A 58149970 (Derwent Abstract AN 83-786760) discloses aqueous vinyl ester/ethylene copolymer dispersions as adhesives for bonding hydrophobic materials, the vinyl esters used preferably being vinyl acetate, vinyl propionate, vinyl laurate and VeoVa10(copyright). Adhesive blends of alkylphenolic resins and aqueous vinyl esters/ethylene copolymer dispersions are known from JP-A 57207662 (Derwent Abstract AN 83-11223K), the preferred vinyl esters being vinyl acetate, vinyl propionate and VeoVa10(copyright). The bonds produced with the adhesives of the invention are notable for increased water resistance. JP-A 01126251 (Derwent Abstract AN 89-188346) describes improving the properties of cementious compositions by adding aqueous dispersions of terpolymers of ethylene/vinyl acetate/Versatic acid vinyl esters of 9 to 11 carbon atoms.
EP-A 315278 describes vinyl acetate copolymers with VeoVa9(copyright) or VeoVa10(copyright), EP-A 431656 acrylate copolymers with VeoVa9(copyright) and, optionally, VeoVa10(copyright), EP-A 486110 acrylate-VeoVa10(copyright) copolymers, and EP-A 516202 acrylate-VeoVa5(copyright) copolymers, in each case as coating materials.
The invention provides a process for preparing adhesives having improved adhesion, in the form of their aqueous dispersions or water-redispersible dispersion powders, by emulsion polymerization in the presence of polyvinyl alcohol with or without drying of the dispersion obtainable by this process, which comprises polymerizing a comonomer mixture comprising
a) one or more monomers from the group consisting of the vinyl esters of unbranched and branched carboxylic acids of 1 to 10 carbon atoms, the esters of acrylic acid and methacrylic acid with branched and unbranched alcohols of 1 to 12 carbon atoms, vinylaromatic compounds, vinyl halides and alpha-olefins, and
b) from 0.01 to 50% by weight, based on the overall weight of the comonomers, of a vinyl ester of alpha-branched tertiary monocarboxylic acids of 11 carbon atoms, in the presence of
c) from 0.1 to 15% by weight, based on the overall weight of the comonomers, of polyvinyl alcohol.
The vinyl esters of alpha-branched tertiary monocarboxylic acids of 11 carbon atoms are produced by Shell Chemicals under the name VeoVa(copyright)11 and are vinyl esters of versatic acid.
Based on the overall weight of the comonomers, it is preferred to copolymerize from 2 to 15% by weight of a vinyl ester of alpha-branched tertiary monocarboxylic acids of 11 carbon atoms with one or more monomers from the group consisting of vinyl esters of unbranched or branched alkylcarboxylic acids of 1 to 10 carbon atoms, methacrylates and acrylates of unbranched or branched alcohols of 1 to 12 carbon atoms, dienes such as butadiene or isoprene, olefins such as ethene or propene, vinylaromatic compounds such as styrene, methylstyrene and vinyltoluene, and vinyl halides such as vinyl chloride, in the stated amounts.
Preferred vinyl esters are vinyl acetate, vinyl propionate, vinyl butyrate, vinyl 2-ethylhexanoate, vinyl laurate, 1-methylvinyl acetate, vinyl pivalate, and vinyl esters of alpha-branched monocarboxylic acids of 5 to 10 carbon atoms, such as VeoVa9(copyright), VeoVa10(copyright) (trade names of Shell). Vinyl acetate is particularly preferred.
Preferred methacrylates or acrylates are methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate, t-butyl acrylate, n-butyl methacrylate, t-butyl methacrylate, and 2-ethylhexyl acrylate. Methyl acrylate, methyl methacrylate, n-butyl acrylate and 2-ethylhexyl acrylate are particularly preferred.
If desired, it is also possible to copolymerize from 0.05 to 10.0% by weight, based on the overall weight of the monomers, of auxiliary monomers from the group consisting of ethylenically unsaturated mono- and dicarboxylic acids and their amides, such as acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, acrylamide, and methacrylamide; ethylenically unsaturated sulfonic acids and their salts, preferably vinylsulfonic acid and 2-acrylamidopropanesulfonate, and N-vinylpyrrolidone.
Further examples of auxiliary monomers in the stated amounts are hydrophobicizing and pro-crosslinking alkoxysilane-functional monomers such as acryloxypropyltri(alkoxy)silanes and methacryloxypropyltri(alkoxy)silanes, vinyltrialkoxysilanes and vinylmethyldialkoxysilanes, the alkoxy groups present possibly being, for example, methoxy, ethoxy, methoxyethylene, ethoxyethylene, methoxypropylene glycol ether and ethoxypropylene glycol ether radicals. Preference is given to vinyltriethoxysilane and gamma-methacryloxypropyltriethoxysilane.
Further examples of auxiliary monomers in the stated amounts are additional crosslinkers such as acrylamidoglycolic acid (AGA), methacrylamidoglycolic acid methyl ester (MAGME), N-methylolacrylamide (NMA), N-methylolmethacrylamide (NMMA), N-methylol allyl-carbamate, alkyl ethers of N-methylolacrylamide or N-methylolmethacrylamide and also the isobutoxy ethers or n-butoxy ethers thereof.
The polymer composition is generally chosen so as to give a glass transition temperature Tg of from xe2x88x9230xc2x0 C. to +40xc2x0 C. The glass transition temperature Tg of the polymers can be determined in a known manner by means of Differential Scanning Calorimetry (DSC). The Tg can also be calculated approximately in advance by means of the Fox equation. According to Fox T. G., Bull. Am. Physics Soc. 1, 3, page 123 (1956) it holds that: 1/Tg=x1/Tg1+x2/Tg2+ . . . + xn/Tgn, where xn is the mass fraction (% by weight/100) of the monomer n and Tgn is the glass transition temperature in degrees Kelvin of the homopolymer of the monomer n. Tg values for homopolymers are listed in Polymer Handbook 3rd Edition, J. Wiley and Sons, New York (1989).
Preferred comonomer mixtures comprise the comonomer b) in the stated amount and also:
vinyl acetate,
vinyl acetate and ethylene with from 30 to 95% by weight of vinyl acetate and an ethylene content of from 1 to 60% by weight;
ethylene and vinyl chloride with an ethylene content of from 1 to 40% by weight and a vinyl chloride content of from 20 to 90% by weight;
vinyl acetate and further vinyl esters with from 30 to 75% by weight of vinyl acetate and from 1 to 50% by weight of one or more copolymerizable vinyl esters from the group consisting of vinyl laurate, vinyl pivalate, vinyl 2-ethylhexanoate, and vinyl esters of an alpha-branched carboxylic acid of 5 to 10 carbon atoms, with or without from 1 to 40% by weight of ethylene in addition;
vinyl acetate and acrylates with from 30 to 90% by weight of vinyl acetate and from 1 to 60% by weight of acrylate, especially n-butyl acrylate or 2-ethylhexyl acrylate, with or without from 1 to 40% by weight of ethylene in addition;
vinyl acetate and acrylates with from 30 to 75% by weight of vinyl acetate and from 1 to 30% by weight of acrylate, especially n-butyl acrylate or 2-ethylhexyl acrylate, with or without from 1 to 40% by weight of ethylene in addition;
n-butyl acrylate or 2-ethylhexyl acrylate;
methyl methacrylate and n-butyl acrylate and/or 2-ethylhexyl acrylate;
vinyl chloride and acrylates, especially n-butyl acrylate or 2-ethylhexyl acrylate;
styrene and butadiene with a styrene content of from 10 to 70% by weight;
styrene and acrylates such as n-butyl acrylate or 2-ethylhexyl acrylate with a styrene content of in each case from 10 to 70% by weight.
The percentages by weight above add up, together with the proportion of comonomer b) and, if appropriate, the proportion of auxiliary monomer, to 100% by weight.
Maximum preference is given to the copolymerization of vinyl acetate and from 2 to 15% by weight, based on the overall weight of the comonomers, of a vinyl ester of alpha-branched tertiary monocarboxylic acids of 11 carbon atoms, and to the copolymerization of from 30 to 95% by weight of vinyl acetate, from 1 to 60% by weight of ethylene as predominant monomers, and from 2 to 15% by weight, based on the overall weight of the comonomers, of a vinyl ester of alpha-branched tertiary monocarboxylic acids of 11 carbon atoms, it being possible in said cases, if desired, for the above-described auxiliary monomers to be copolymerized as well.
The preparation, in accordance with the emulsion polymerization process, is conducted in conventional reactors or pressure reactors in a temperature range from 30xc2x0 C. to 80xc2x0 C. and is initiated by the methods commonly employed for emulsion polymerization. In the case of the copolymerization of gaseous monomers such as ethylene, it is preferred to operate at a pressure of from 5 to 85 barabs.. Initiation is effected by means of the customary, at least partially water-soluble free-radical initiators, which are employed preferably in amounts of from 0.01 to 3.0% by weight based on the overall weight of the monomers. Examples of such initiators are sodium persulfate, hydrogen peroxide, t-butyl peroxide, t-butyl hydroperoxide; potassium peroxodiphosphate, and azobisisobutyronitrile. If desired, said free-radical initiators can also be combined, in a known manner, with from 0.01 to 0.5% by weight, based on the overall weight of the monomers, of reducing agents. Suitable examples are alkali metal formaldehyde-sulfoxylates and ascorbic acid. In the case of redox initiation, it is preferred to add one or both redox catalyst components during the polymerization.
The copolymerization takes place in the presence of polyvinyl alcohol as a protective colloid, generally, in an amount of from 0.1 to 15% by weight, preferably from 1 to 10% by weight, based on the overall weight of the monomers. Preference is given to partially hydrolyzed polyvinyl alcohols containing from 75 to 100 mol %, with particular preference from 78 to 95 mol %, of vinyl alcohol units and having a Hxc3x6ppler viscosity of from 3 to 60 mPas (4% strength aqueous solution, Hxc3x6ppler method in accordance with DIN 53015).
The copolymerization is preferably conducted without emulsifier; alternatively, it is possible to employ any of the emulsifiers commonly used for emulsion polymerization. Suitable emulsifiers include anionic, cationic and nonionic emulsifiers, in their customary amounts of from 0.5 to 6% by weight, based on the overall weight of the monomers. The desired pH range for the polymerization, which generally lies between 2.5 and 10, preferably between 3 and 8, can be established in a known manner by means of acids, bases and customary buffer salts such as alkali metal phosphates or alkali metal carbonates. In order to establish the molecular weight, it is possible to use the commonly employed regulators in the polymerization, examples being mercaptans, aldehydes and chlorinated hydrocarbons.
Irrespective of the chosen polymerization process, the polymerization can be conducted batchwise or continuously, with or without the use of seed lattices, with an initial charge comprising some or all of the constituents of the reaction mixture, or with an initial charge comprising a portion of some or all of the constituents of the reaction mixture, with the remaining portion(s) being metered in subsequently, or by the metering process without an initial charge. The solids content of the dispersion obtainable in this way is from 20 to 70%.
The dispersion can be dried by means of spray drying, freeze drying or fluidized-bed drying. Preference is given to spray drying in customary spray drying plants, in which spraying can be effected by means of single-, dual- or multi-substance nozzles or with a rotating disk. The exit temperature is generally chosen to be in the range from 55xc2x0 C. to 100xc2x0 C., preferably from 65xc2x0 C. to 90xc2x0 C., depending on the plant, on the Tg of the resin and on the desired degree of drying.
For spray drying, the dispersion of the polymer, with a solids content of preferably from 20% to 70%, is sprayed together with protective colloids as an atomization aid and dried. Examples of protective colloids which can be employed in this procedure are partially hydrolyzed polyvinyl alcohols, polyvinylpyrrolidones, starches, melamineformaldehydesulfonates, and naphthaleneformaldehydesulfonates. In this process step, it is preferred to add from 5 to 20% by weight of protective colloid, based on the polymer. If desired, additives may also be added to the polymer powder. Examples of modifying additives are antiblocking agents, dyes, pigments, plasticizers, film-forming auxiliaries, antifoams, catalysts, rheological assistants, thickeners, tackifiers and emulsifiers.
The aqueous dispersions and water-redispersible redispersion powders of the invention are suitable as adhesives for bonding various substrates: for example, wood, cardboard, paper and fiber materials. The dispersions and powders are particularly useful as paper adhesives, packaging adhesives, bookbinding adhesives, wood adhesives, parquet adhesives, as adhesives for fiber materials and as adhesives in the architectural sector, examples being tile adhesives.
For these applications the dispersions and/or powders can be modified with the appropriate additives. Suitable additives include fillers such as chalk or gypsum. It is also possible to add wetting agents, dispersants, thickeners, defoamers and/or preservatives.
A surprising feature is the improved adhesion of the adhesive raw materials to a variety of substrates, which is observed even when small amounts of VeoVa11(copyright) are incorporated into the copolymer. All the more surprising is that, with markedly increased adhesion, there is only a slight decrease in the cohesion of the bond. The use of the VeoVa9(copyright) or VeoVa10(copyright) fails to achieve any improvement in the adhesion.
The examples which follow serve to illustrate the invention: